The present invention relates to a spin exciting method, a magnetic resonance imaging method, and a magnetic resonance imaging system. More specifically, it relates to a spin exciting method, a magnetic resonance imaging method, and a magnetic resonance imaging system for producing tomographic images of multiple slices. Herein, spins in an object, in which two kinds of magnetic resonance signals having different frequencies are induced based on chemical shifts, that have one of two peak resonance signals are saturated, and spins that have the other peak resonant frequency are utilized.
In magnetic resonance imaging (MRI) systems, an object of imaging is carried into a bore of a magnet system, that is, an imaging space in which a static magnetic field is created. Magnetic field gradients and a high-frequency magnetic field are applied to the object in order to excite spins in the object. This causes the spins to induce a magnetic resonance (MR) signal. An image is reconstructed based on the received MR signal.
The MR signal utilized for imaging is induced by spins possessed by protons of hydrogen nuclei. The protons exist in water that is a major component of a living body, and are therefore preferably adopted as a signal source necessary to visualize the living body through magnetic resonance imaging.
Since protons also exist in fat, fat also induces a magnetic resonance signal. Due to chemical shifts, the frequency of the magnetic resonance signal induced by spins possessed by the protons of fat is different from the frequency of the one induced by spins possessed by the protons of water. The resonant frequency of fat is lower by an equivalent of approximately 3.5 ppm than the resonant frequency of water.
By utilizing the difference in a resonant frequency, water alone is visualized but fat is not, or fat alone is visualized but water is not. In order to visualize water alone, processing of preventing fat from inducing a magnetic resonance signal is performed in advance. Moreover, in order to visualize fat alone, processing of preventing water from inducing a magnetic resonance signal is performed in advance.
The processing is referred to as saturation, wherein spins possessed by the protons of fat or water are excited and fully diffused so that the spins will not respond to subsequent excitation. Excitation of spins for saturation is initiated with a radio-frequency (RF) signal whose frequency agrees with the resonant frequency of fat or water.
If a static magnetic field is inhomogeneous in strength from site to site in an imaging space, resonant frequencies of spins possessed by protons are uncertain depending on a site. Depending on the degree of uncertainty, the spins to be used for imaging may be saturated. This effect of the uncertainty gets especially outstanding in multiple slice imaging.